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SubjectRe: [PATCH 17/30] Documentation: kconfig: document a new Kconfig macro language
From
Date
On 04/12/18 22:06, Masahiro Yamada wrote:
> Add a document for the macro language introduced to Kconfig.
>
> Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
> ---
>
> Changes in v3: None
> Changes in v2: None
>
> Documentation/kbuild/kconfig-macro-language.txt | 179 ++++++++++++++++++++++++
> MAINTAINERS | 2 +-
> 2 files changed, 180 insertions(+), 1 deletion(-)
> create mode 100644 Documentation/kbuild/kconfig-macro-language.txt
>
> diff --git a/Documentation/kbuild/kconfig-macro-language.txt b/Documentation/kbuild/kconfig-macro-language.txt
> new file mode 100644
> index 0000000..1f6281b
> --- /dev/null
> +++ b/Documentation/kbuild/kconfig-macro-language.txt
> @@ -0,0 +1,179 @@
> +Concept
> +-------
> +
> +The basic idea was inspired by Make. When we look at Make, we notice sort of
> +two languages in one. One language describes dependency graphs consisting of
> +targets and prerequisites. The other is a macro language for performing textual
> +substitution.
> +
> +There is clear distinction between the two language stages. For example, you
> +can write a makefile like follows:
> +
> + APP := foo
> + SRC := foo.c
> + CC := gcc
> +
> + $(APP): $(SRC)
> + $(CC) -o $(APP) $(SRC)
> +
> +The macro language replaces the variable references with their expanded form,
> +and handles as if the source file were input like follows:
> +
> + foo: foo.c
> + gcc -o foo foo.c
> +
> +Then, Make analyzes the dependency graph and determines the targets to be
> +updated.
> +
> +The idea is quite similar in Kconfig - it is possible to describe a Kconfig
> +file like this:
> +
> + CC := gcc
> +
> + config CC_HAS_FOO
> + def_bool $(shell $(srctree)/scripts/gcc-check-foo.sh $(CC))
> +
> +The macro language in Kconfig processes the source file into the following
> +intermediate:
> +
> + config CC_HAS_FOO
> + def_bool y
> +
> +Then, Kconfig moves onto the evaluation stage to resolve inter-symbol
> +dependency, which is explained in kconfig-language.txt.
> +
> +
> +Variables
> +---------
> +
> +Like in Make, a variable in Kconfig works as a macro variable. A macro
> +variable is expanded "in place" to yield a text string that may then expanded

may then be expanded

> +further. To get the value of a variable, enclose the variable name in $( ).
> +As a special case, single-letter variable names can omit the parentheses and is

and are

> +simply referenced like $X. Unlike Make, Kconfig does not support curly braces
> +as in ${CC}.
> +
> +There are two types of variables: simply expanded variables and recursively
> +expanded variables.
> +
> +A simply expanded variable is defined using the := assignment operator. Its
> +righthand side is expanded immediately upon reading the line from the Kconfig
> +file.
> +
> +A recursively expanded variable is defined using the = assignment operator.
> +Its righthand side is simply stored as the value of the variable without
> +expanding it in any way. Instead, the expansion is performed when the variable
> +is used.
> +
> +There is another type of assignment operator; += is used to append text to a
> +variable. The righthand side of += is expanded immediately if the lefthand
> +side was originally defined as a simple variable. Otherwise, its evaluation is
> +deferred.
> +
> +
> +Functions
> +---------
> +
> +Like Make, Kconfig supports both built-in and user-defined functions. A
> +function invocation looks much like a variable reference, but includes one or
> +more parameters separated by commas:
> +
> + $(function-name arg1, arg2, arg3)
> +
> +Some functions are implemented as a built-in function. Currently, Kconfig
> +supports the following:
> +
> + - $(shell command)
> +
> + The 'shell' function accepts a single argument that is expanded and passed
> + to a subshell for execution. The standard output of the command is then read
> + and returned as the value of the function. Every newline in the output is
> + replaced with a space. Any trailing newlines are deleted. The standard error
> + is not returned, nor is any program exit status.
> +
> + - $(warning text)
> +
> + The 'warning' function prints its arguments to stderr. The output is prefixed
> + with the name of the current Kconfig file, the current line number. It

file and the current line number. It

> + evaluates to an empty string.
> +
> + - $(info text)
> +
> + The 'info' function is similar to 'warning' except that it sends its argument
> + to stdout without any Kconfig name or line number.

Are current Kconfig file name and line number available so that someone can
construct their own $(info message) messages?

> +
> +A user-defined function is defined by using the = operator. The parameters are
> +referenced within the body definition with $1, $2, etc. (or $(1), $(2), etc.)
> +In fact, a user-defined function is internally treated as a recursive variable.

so the difference is just whether there are arguments?
A recursive variable does not have arguments and a function always has at least
one argument?


> +
> +A user-defined function is referenced in the same way as a built-in function:
> +
> + $(my_func, arg0, arg1, arg2)

Syntax given above is:
+ $(function-name arg1, arg2, arg3)

with no comma after the function name. Which is it?

> +
> +Note 1:
> +There is a slight difference in the whitespace handling of the function call
> +between Make and Kconfig. In Make, leading whitespaces are trimmed from the
> +first argument. So, $(info FOO) is equivalent to $(info FOO). Kconfig keeps
> +any leading whitespaces except the one right after the function name, which
> +works as a separator. So, $(info FOO) prints " FOO" to the stdout.
> +
> +Note 2:
> +In Make, a user-defined function is referenced by using a built-in function,
> +'call', like this:
> +
> + $(call my_func, arg0, arg1, arg2)
> +
> +However, Kconfig did not adopt this form just for the purpose of shortening the
> +syntax.
> +
> +
> +Caveats
> +-------
> +
> +A variable (or function) can not be expanded across tokens. So, you can not use

cannot cannot

> +a variable as a shorthand for an expression that consists of multiple tokens.
> +The following works:
> +
> + RANGE_MIN := 1
> + RANGE_MAX := 3
> +
> + config FOO
> + int "foo"
> + range $(RANGE_MIN) $(RANGE_MAX)
> +
> +But, the following does not work:
> +
> + RANGES := 1 3
> +
> + config FOO
> + int "foo"
> + range $(RANGES)
> +
> +A variable can not be expanded to any keyword in Kconfig. The following does

cannot

> +not work:
> +
> + MY_TYPE := tristate
> +
> + config FOO
> + $(MY_TYPE) "foo"
> + default y
> +
> +Obviously from the design, $(shell command) is expanded in the textual
> +substitution phase. You can not pass symbols to the 'shell' function.

cannot

> +The following does not work as expected.
> +
> + config ENDIAN_OPTION
> + string
> + default "-mbig-endian" if CPU_BIG_ENDIAN
> + default "-mlittle-endian" if CPU_LITTLE_ENDIAN
> +
> + config CC_HAS_ENDIAN_OPTION
> + def_bool $(shell $(srctree)/scripts/gcc-check-option ENDIAN_OPTION)
> +
> +Instead, you can do like follows so that any function call is statically
> +expanded.
> +
> + config CC_HAS_ENDIAN_OPTION
> + bool

fix indentation?

> + default $(shell $(srctree)/scripts/gcc-check-option -mbig-endian) if CPU_BIG_ENDIAN
> + default $(shell $(srctree)/scripts/gcc-check-option -mlittle-endian) if CPU_LITTLE_ENDIAN


--
~Randy

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